John V the Conqueror (in Breton Yann IV, in French Jean IV) (1339 – November 1, 1399), was Duke of Brittany and Count of Montfort, from 1345 until his death. He was son of Duke John of Montfort and Joanna of Flanders.

The first part of his rule was tainted by the Breton War of Succession, fought against his cousin Joanna of Penthièvre and her husband Charles of Blois. After his father's death, his mother took him to England to ask for the aid of Edward III. His mother was declared insane and imprisoned in Tickhill Castle in 1343. He was taken in to the King's household afterwards. In 1364, John V managed to win an important victory against the House of Blois in the battle of Auray, with the help of the English army. His rival Charles was killed in battle and Joanna forced to sign the Treaty Guérande on April 12, 1365. In the terms of the treaty, Joanna gave up her rights to Brittany and recognized John IV as sole master of the duchy. Surprisingly, John V declared himself a vassal to king Charles V of France, not to Edward III of England who helped him to become duke, and whose daughter had been John's first wife. Nevertheless, the French exerted pressure over Brittany and the local nobles and forced John V to exile between 1373 and 1379.

John V the Conqueror KG (in Breton Yann V, in French Jean V) (1339 – 1 November 1399) was Duke of Brittany and Count of Montfort, from 1345 until his death.

Numbering

He was son of John de Montfort and Joanna of Flanders. His father claimed the title John IV, Duke of Brittany, but was largely unable to enforce his claim for more than a brief period. Because his father's claim to the title was disputed, the subject of this article has often been numbered John IV, while his father has been referred to as simply "John de Montfort". He is still numbered John IV by some historians and is still more commonly known by that designation (Jean IV) in France, since the French monarchy, unlike the English, never acknowledged his father's title.

Conquest

The first part of his rule was tainted by the Breton War of Succession, fought by his father against his cousin Joanna of Penthièvre and her husband Charles of Blois. With French military support Charles was able to control most of Brittany. After his father's death, John's mother Jeanne attempted to continue the war in the name of her baby son. She became known as "Jeanne la Flamme" for her fiery personality. However, she was eventually forced to retreat with her son to England to ask for the aid of Edward III. She was later declared insane and imprisoned in Tickhill Castle in 1343. John and his sister Joan of Brittany were taken into the King's household afterwards.

John returned to Brittany to enforce his claim, with English help. In 1364, John managed to win a decisive victory against the House of Blois in the Battle of Auray, with the support of the English army. His rival Charles was killed in the battle and Charles's widow Joanna was forced to sign the Treaty Guérande on 12 April 1365. In the terms of the treaty, Joanna gave up her rights to Brittany and recognized John as sole master of the duchy.

Power struggles

Having achieved victory with English support (and having married into the English royal family), Jean was constrained to confirm several English barons in positions of power within Brittany, especially as controllers of strategically important strongholds in the environs of the port of Brest, which gave the English military access to the peninsula and which took revenue from Brittany to the English crown. This English powerbase in Brittany was resented by the Breton aristocrats and the French monarchy, as was John's use of English advisors. However, John V declared himself a vassal to king Charles V of France, not to Edward III of England. This gesture did not placate his critics, who saw the presence of rogue English troops and lords as destabilizing. Faced with the defiance of the Breton nobility, John was unable to muster military support against Charles V, who took the opportunity to exert pressure over Brittany. Without local support, in 1373 Jean was forced into exile once more in England.

However, Charles V made the mistake of attempting to completely annex the duchy to France. Bertrand de Guesclin was sent to make the duchy submit to the French king by force of arms in 1378. The barons revolted against the annexation and invited John V back from exile in 1379. He landed in Dinard and took control of the duchy once more with the support of local barons. An English army under Thomas of Woodstock, 1st Duke of Gloucester was landed at Calais and marched towards Nantes to take control of the city. However, John reconciled with the new French king Charles VI and paid off the English troops to avoid a confrontation. He ruled his duchy thereafter in peace with the French and English crowns for over a decade, maintaining contact with both, but minimising open links to England. He also managed to extricate Brest from English control in 1397 using diplomatic pressure and financial inducements.

Clisson affair

In 1392 an attempt was made to kill Olivier de Clisson, the Constable of France, who was an old enemy of the duke's. The attacker, Pierre de Craon, fled to Brittany. John was assumed to be behind the plot, and Charles VI took the opportunity to attack Brittany once more. Accompanied by the Constable, he marched on Brittany, but before he reached the duchy the king was seized with madness. Relatives of Charles VI blamed Clisson, and instituted legal proceedings against him to undermine his political position. Stripped of his status as Constable, Clisson now took refuge in Brittany himself, and was reconciled with John (1397), becoming a close advisor to the duke.